Despite its simplicity, the bacterium has a well-developed cell structure that contributes to its distinct biological features and pathogenicity. Many structural properties of bacteria are found nowhere else in archaea or eukaryotes. The cell structure of bacteria has been widely investigated, revealing many biochemical concepts that have since been applied to other creatures, due to their relative simplicity versus larger organisms and the ease with which they may be modified experimentally.
Bacterial morphology
Bacterial morphology is perhaps the most fundamental structural characteristic (shape). Here are a few examples:
the coccus (circle or spherical)
Bacillus coagulans (rod-like)
bacillus coccobacillus (between a sphere and a rod)
spiralling (corkscrew-like)
filiformes (elongated)
The form of a bacterial cell is normally characteristic of that species, but it can vary depending on growth conditions. Some bacteria (such as Caulobacter) have complicated life cycles that include the creation of stalks and appendages, while others produce sophisticated structures that contain reproductive spores (e.g. Myxococcus, Streptomyces). When bacteria are cultured on Petri plates, they generate unique colony morphologies and diverse cell morphologies when studied under a microscope.
Cell wall
The cell membrane and the cell wall make up the cell envelope. The bacterial cell wall, like that of other species, offers structural stability to the cell. The fundamental function of the cell wall in prokaryotes is to protect the cell from internal turgor pressure, which is induced by significantly higher concentrations of proteins and other chemicals inside the cell than outside.
The presence of peptidoglycan just outside the cell membrane distinguishes the bacterial cell wall from that of all other species. The polysaccharide backbone of peptidoglycan is made up of equal numbers of N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) residues. The rigidity of the bacterial cell wall and the determination of cell shape are both determined by peptidoglycan. It is porous and not regarded as a permeability barrier for small substrates because of its porous nature.
Plasma membrane
The plasma membrane, also known as the bacterial cytoplasmic membrane, is made up of a phospholipid bilayer that performs all of the tasks of a cell membrane, including acting as a permeability barrier for most molecules and serving as a transport pathway for chemicals into the cell.
In addition to these activities, prokaryotic membranes serve as the source of a proton motive force, which helps to conserve energy. Unlike eukaryotes, bacterial membranes generally lack sterols (with a few exceptions, such as Mycoplasma and methanotrophs). Many microorganisms, however, have structurally comparable chemicals called hopanoids, which are thought to serve the same purpose. Bacteria, unlike eukaryotes, can have a wide range of fatty acids in their membranes.
Fimbriae and pili
Fimbriae (also known as “attachment pili”) are protein tubes that protrude from the outer membrane of numerous Proteobacteria species. They are usually short in length and can be found in large numbers all over a bacterial cell’s surface. Fimbriae are structures that help bacteria connect to a surface (for example, to form a biofilm) or to other cells (e.g. animal cells during pathogenesis).
Fimbriae are used by a few species (such as Myxococcus) to promote the building of multicellular structures such as fruiting bodies. Pili have a shape that is similar to fimbriae, but they are much longer and occur in smaller numbers on the bacterial cell. Pili are involved in bacterial conjugation and are referred to as “sex pili” or “conjugation pili.” Type IV pili (non-sex pili) help bacteria grip surfaces as well.
S-layers
An S-layer (surface layer) is a cell surface protein layer that functions as the cell wall in a variety of bacteria and archaea. S-layers are made up of a two-dimensional array of proteins with a crystalline appearance, albeit the symmetry varies by species. S-layers’ specific function is uncertain, however they are thought to act as a partial permeability barrier for large substrates.
Glycocalyx
Glycocalyx is a type of extracellular polymer secreted by many bacteria outside of their cell walls. Polysaccharides and protein are common components of these polymers. Capsules are generally impermeable structures that are resistant to dyes like India ink. They’re anti-phagocytosis and anti-desiccation features that assist bacteria survive. The slime layer aids in the formation of biofilms by allowing bacteria to bind to other cells or inanimate surfaces. Slime layers can also be used as a cell’s food reserve.
Intracellular (internal) structures
The internal characteristics of the bacterial cell are exceedingly simple in comparison to those of eukaryotes. Organelles do not exist in bacteria in the same way that they do in eukaryotes. Instead, in all bacteria, the chromosome and maybe ribosomes are the only plainly visible internal structures. Specialized kinds of bacteria do, however, exhibit more sophisticated internal structures, some of which are mentioned below.
The bacterial DNA and plasmids
Unlike eukaryotes, bacteria’s DNA is not contained within a membrane-bound nucleus, but rather exists in the cytoplasm. This means that translation, transcription, and DNA replication all take place within the same compartment and can interact with other cytoplasmic components, most notably ribosomes.
Ribosomes and other multiprotein complexes
The ribosome, which is the location of protein synthesis in all living creatures, is the most abundant intracellular structure in most bacteria. Prokaryotes have 70S ribosomes (where S stands for Svedberg units), but eukaryotes have larger 80S ribosomes in their cytoplasm. The 50S and 30S subunits form the 70S ribosome. The 23S and 5S rRNA are found in the 50S subunit, while the 16S rRNA is found in the 30S subunit.
Endospores
The development of endospores is perhaps the most well-known bacterial adaptation to stress. Endospores are bacterial survival structures that are highly resistant to a wide range of chemical and environmental challenges, allowing bacteria to survive in conditions that would otherwise kill them in their vegetative state.
Conclusion
Bacteria are prokaryotes, without well-defined nuclei and membrane-bound organelles, and with chromosomes made of a single closed DNA circular. They appear in different shapes and sizes, from minute spheres, cylinders and spiral threads, to flagellated rods, and filamentous chains.